Amplifiers used in multi-channel applications, such as cellular radio and Personal Communication System (PCS) transceivers, typically produce output signals having desired signals and undesirable intermodulation products created by the amplifiers. While the intermodulation products may be at a very low level relative to the desired signals, the level of intermodulation products may be high enough to impair transmission or exceed some acceptable threshold or standard. The level of the intermodulation product created by the amplifiers is determined, in part, by the characteristics of the amplifiers, which is dependent on a variety of parameters, such as temperature, supply voltage, signal level, age, etc.
To reduce the level of the intermodulation products (i.e., linearize), the amplifier can be incorporated into a circuit having a linearizer. Various linearizers may be employed to linearize the amplifiers. Such linearizers incorporate well-known linearization techniques, such as feedforward, feedback, pre-distortion and post-distortion. Linearizing an amplifier involves adjusting the linearizers to match or track the characteristics of the amplifier being linearized.
Adjusting the linearizers to match or track the characteristics of the amplifier is relatively straightforward under static conditions, e.g., constant temperature, supply voltage, signal level, etc., as is well known in the art. Under non-static conditions, however, adjusting the linearizer to match or track the characteristics of the amplifier is more complex. Each time a parameter affecting the characteristics of the amplifier changes, the linearizer would need to be adjusted to compensate.
There are two fundamentally different approaches to adjusting linearizers to compensate for non-static conditions: open-loop and closed-loop control or compensation. Open-loop control involves measuring changes in the parameters affecting the characteristics of the amplifier (e.g., temperature), and using the measured parameter changes to adjust the linearizer to match or track the characteristics of the amplifier. The effectiveness of open-loop control depends upon how well the amplifier can be characterized (with respect to the parameters which affect the characteristics of the amplifier) and the range over which parameters can vary. For cellular radio and PCS transmitters, the effectiveness of open-loop control is generally inadequate because amplifier characteristics change with channel loading (i.e., number of carriers) and time.
Closed-loop control involves measuring the effects of changes in the parameters affecting the characteristics of the amplifier, and using the measured effects to adaptively adjust the linearizer to match or track the characteristics of the amplifiers. For multi-channel applications (e.g., cellular radio and PCS transceivers), the signal levels of the intermodulation products are measured to indicate the effects of the parameter changes. The intermodulation products may be measured relatively easy using spectrum analyzers. However, spectrum analyzers are, in most cases, prohibitively expensive. Another manner of measuring the intermodulation products involves using less expensive hardware associated with linearizers, such as pilot tone generators and receivers.
Measuring the intermodulation products using the pilot tone generators and receivers involves injecting pilot tones (using the pilot tone generators) into various points of the circuit (comprising the linearizer and the amplifier) to effectively simulate the intermodulation products. The signal levels of the pilot tones are then measured at the output of the circuit using the pilot tone receivers. The signal levels of the pilot tones at the output will provide an indication of the levels of the intermodulation products, which can be used to adaptively adjust the linearizer to match or track the characteristics of the amplifier.
Although the pilot tone generators and receivers are relatively inexpensive compared to the spectrum analyzers, the pilot tone generators and receivers do involve extra cost to the circuit. In addition, the pilot tone generators and receivers have associated performance problems. First, it is not always obvious where to inject the pilot tones. The pilot tones should be injected where they will not interfere with the desired signals, and the pilot tones should be injected at points that will yield performance representative for the entire frequency band. Second, the pilot tones should be reduced to levels at the output (of the circuit) commensurate to those of the intermodulation products. This means that the levels of the pilot tones cannot be arbitrarily high, and may mean that the pilot tone receivers will have to be correspondingly more sophisticated to cancel the pilot tones at the output.
Accordingly, there exists a need for linearizing amplifiers by measuring the intermodulation products at the output of the linearized amplifier using minimum additional hardware without the performance problems associated with pilot tones.